265Near Metalloids...............................
The concept of a class of elements intermediate between metals and nonmetals is
sometimes extended to include elements that most chemists, and related science
professionals, would not ordinarily recognize as metalloids. In 1935, Fernelius and Robey
allocated carbon, phosphorus, selenium, and iodine to such an intermediary class of
elements, together with boron, silicon, arsenic, antimony, tellurium and polonium. They
also included a placeholder for the missing element 85 (astatine), five years ahead of its
synthesis in 1940.
They excluded germanium from their considerations as it was still then regarded as a
poorly conducting metal. In 1954, Szabó & Lakatos counted beryllium and aluminum in
their list of metalloids, as well as boron, silicon, germanium, arsenic, antimony, tellurium,
polonium and astatine. In 1957, Sanderson recognized carbon, phosphorus, selenium,
and iodine as part of an intermediary class of elements with 'certain metallic properties',
together with boron, silicon, arsenic, tellurium, and astatine. Germanium, antimony and
polonium were classified by him as metals. More recently, in 2007, Petty included carbon,
phosphorus, selenium, tin and bismuth in his list of metalloids, as well as boron, silicon,
germanium, arsenic, antimony, tellurium, polonium and astatine.
Elements such as these are occasionally called or described as, near-metalloids, or the
like. They are located near the elements commonly recognized as metalloids, and usually
classified as either metals or nonmetals. Metals falling into this loose category tend to
show 'odd' packing structures, marked covalent chemistry (molecular or polymeric), and
amphoterism.
Aluminum, tin and bismuth are examples. They are also referred to as (chemically) weak
metals, poor metals, post-transition metals, or semimetals (in the aforementioned sense
of metals with incomplete metallic character). These classification groupings generally
cohabit the same periodic table territory but are not necessarily mutually inclusive.
Nonmetals in the 'near-metalloid' category include carbon, phosphorus, selenium and
iodine. They exhibit metallic luster, semiconducting properties and bonding or valence
bands with delocalized character.
This applies to their most thermodynamically stable forms under ambient conditions:
carbon as graphite; phosphorus as black phosphorus; and selenium as grey selenium.
These elements are alternatively described as being 'near metalloidal', showing
metalloidal character, or having metalloid-like or some metalloid(al) or metallic properties.
Allotropes
Some allotropes of the elements exhibit more pronounced metallic, metalloidal or
nonmetallic behavior than others. For example, the diamond allotrope of carbon is clearly
nonmetallic. The graphite allotrope however displays limited electrical conductivity more
characteristic of a metalloid. Phosphorus, selenium, tin, and bismuth also have allotropes
that display borderline or either metallic or nonmetallic behavior.